1. Field of the Invention
The present invention relates generally to lithographic projection systems and more particularly to lithographic projection systems incorporating an intensity distribution sensor.
2. Background of the Related Art
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system also includes components operating according to any of these design types, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d.
The radiation system as well as the projection system generally comprise components for directing, shaping or controlling the projection beam of radiation. Generally, the projection system comprises means to set the numerical aperture (commonly referred to as the xe2x80x9cNAxe2x80x9d) of the projection system. For example, an aperture adjustable NA-diaphragm is provided in a pupil of the projection system. The radiation system typically comprises adjusting means for setting the outer and/or inner radial extent (commonly referred to as "sgr"-outer and "sgr"-inner, respectively) of the intensity distribution upstream of the mask (in a pupil of the radiation system).
In addition, the first and second object tables may be referred to as the xe2x80x9cmask tablexe2x80x9d and the xe2x80x9csubstrate tablexe2x80x9d, respectively. Further, the lithographic apparatus may be of a type having two or more mask tables and/or two or more substrate tables. In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more stages while one or more other stages are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (which may comprise one or more dies) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions which are successively irradiated via the reticle, one at a time. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire reticle pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper or just xe2x80x9cstepperxe2x80x9d. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatus or xe2x80x9cscannerxe2x80x9dxe2x80x94each target portion is irradiated by progressively scanning the reticle pattern under the (slitted) projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallisation, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although the spatial distribution of projection beam radiation at wafer level is measured accurately in a lithographic projection apparatus, the angular distribution is generally not monitored. Consequently, properties of the performance of the projection system, such as angular dependent lens transmission are unknown. Similarly, the projection system comprises a pupil. The spatial intensity distribution at this pupil is related to the angular intensity distribution at the reticle and the wafer. The spatial intensity distribution at said pupil is, in practice, very difficult to measure in situ and sufficiently fast (such as to not substantially impair the number of substrates that can be processed per unit of time), but potentially contains valuable information relating to the alignment, performance and optimisation of the lithographic projection apparatus. Conventional techniques which measure the intensity distribution at wafer level or reticle level do not enable an assessment of specific (residual) positional and angular alignment errors of corresponding specific optical components. These techniques can also be highly dependent on the "sgr"-outer and "sgr"-inner settings, which means that many measurements must be taken and an alignment procedure for said specific optical components is time-consuming.
Accordingly, the present invention provides a method of operating a lithographic projection apparatus comprising:
a radiation system, for providing a projection beam of radiation;
a first object table for holding a mask at a mask plane;
a second object table for holding a substrate at a substrate plane; and
a projection system for imaging at least a portion of the mask onto target portions of the substrate;
the method characterized by comprising the steps of:
forming at least one spot of radiation from at least a portion of said projection beam in said apparatus;
providing at least one radiation sensor, embodied for real time electronic analysis of an intensity distribution;
measuring, with said at least one sensor, the spatial variation in intensity of defocused radiation from said spot or from an image of said spot; and
determining properties of said apparatus from information obtained in said measuring step.
This method enables properties of the pupil in the projection system to be determined and the angular intensity distribution property of the apparatus to be determined.
The or each spot may be formed at at least one of the mask plane and the substrate plane.
The or each spot may be formed by using a substantially transmissive or substantially radiation blocking pinhole.
The method may further comprise generating radiation at particular angles using, at the or each spot, at least one radiation manipulation effect selected from the group of radiation manipulation effects comprising diffraction, scattering and diffusion of radiation.
By diffracting radiation at large angles the NA-diaphragm size and shape can be determined. Instead of diffraction effects, scattering or diffusion of radiation can be used to generate radiation at large angles. Scattering of radiation occurs, for example, when the projection beam traverses a rough surface. Similarly, diffusion of radiation occurs, for example, when the projection beam traverses a translucent element.
The properties of said apparatus may include at least one of:
for a pupil in said apparatus, the shape, symmetry, fine structure and/or centering of the angular intensity distribution with respect to an NA-diaphragm,
the shape and/or size of an NA-diaphragm in said apparatus,
the angular dependence of radiation transmission in said projection system,
the angular intensity distribution at the mask plane and/or substrate plane,
the alignment of optical components in said apparatus, in particular the positional and angular alignment of the source of radiation, and
any one or more of the above properties at different positions in the mask plane and/or substrate plane, and/or at different illumination settings of said apparatus.
The method may further comprise the step of adjusting the apparatus to correct or compensate for deviation from optimal of any of the determined properties. Said step of adjusting the apparatus may substantially reduce image anomalies of a projected pattern, improve the accuracy of the positioning in registry of stacked layers of a device (referred to hereinafter as the xe2x80x9coverlay performancexe2x80x9d). Said step may also improve the uniformity, within a target portion, of the critical dimension. The critical dimension is the dimension of a feature or features, such as the gate width of a transistor, in which variations will cause undesirable variation in physical properties of the feature. Said uniformity of the critical dimension is referred to in this text as xe2x80x9cCD uniformityxe2x80x9d.
The invention also provides a lithographic projection apparatus for imaging a mask pattern in a mask onto a substrate provided with a radiation-sensitive layer, the apparatus comprising:
a radiation system, for providing a projection beam of radiation;
a first object table for holding a mask at a mask plane;
a second object table for holding a substrate at a substrate plane; and
a projection system for imaging irradiated portions of the mask onto target portions of the substrate;
characterized by further comprising:
at least one spot formation device for forming at least one spot of radiation from at least a portion of said projection beam in said apparatus; and
at least one radiation sensor, embodied for real time electronic analysis of an intensity distribution, and defocused with respect to the spot or from each respective spot, or image thereof, for measuring the spatial variation in intensity of defocused radiation from the or each spot or image thereof.
In one embodiment, said spot formation device comprises at least one substantially transmissive or substantially radiation blocking pinhole located at one of the mask plane and the substrate plane.
In another embodiment, said or each pinhole further comprises dots with a size approximately of the order of the wavelength of the projection beam of radiation, for diffracting the radiation, which enables radiation at large angles of incidence to be generated.
In another embodiment, in use, the or each sensor is spaced apart from its respective spot by a distance greater than the size of the spot.
These features enable the spot and sensor to act as a pinhole camera, which has a very simple structure, requiring few components, and which is compact.
In one aspect of an embodiment of the invention the size of the spot is approximately 1% of the image field area or less.
The at least one sensor may comprise a photodiode with small detection area or a charge-coupled device (CCD), optionally further comprising a lens. Consequently, the apparatus and method of the invention enable in situ, real time measurements of pupil intensity distribution. For this in situ measurement exposure and processing of a substrate provided with a radiation sensitive resist layer is no longer necessary. This makes the measurements rapid, quantitative and independent of resist-processing.
Likewise, said at least one sensor may be scanable to enable an image of the pupil to be obtained and enables properties of the apparatus to be determined at different field positions.
The apparatus may further comprise a calculation unit for determining properties of said apparatus from the measurements taken by the or each sensor. This enables the properties to be obtained on-line and in real time.
The apparatus may further comprise actuators for adjusting said apparatus to correct and/or compensate for deviation from optimal of any of the determined properties based on signals from said calculating unit. Thus the adjustment process is simplified and automated.
The invention further provides a method of manufacturing a device comprising operating a lithographic projection apparatus as defined above according to the method of the invention, and a device manufactured thereby.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms illumination radiation and illumination beam are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV radiation (extreme ultraviolet radiation, e.g. having a wavelength in the range 5-20 nm).